Asteroids

On the first day of January 1801,
Giuseppe Piazzi
discovered an object which he first thought was a new comet.
But after its orbit was better determined it was clear that it was not a comet but more like a small planet. Piazzi named it Ceres, after the Sicilian goddess of grain. Three other small bodies were discovered in the next few years (Pallas, Vesta, and Juno). By the end of the 19th century there were several hundred.

Several hundred thousand asteroids have been discovered and given provisional designations so far.
Thousands more are discovered each
year. There are undoubtedly hundreds of thousands more that are too small
to be seen from the Earth.
There are 26 known asteroids larger than 200 km
in diameter. Our census of the largest ones is now fairly complete: we probably
know 99% of the asteroids larger than 100 km in diameter. Of those in the 10 to
100 km range we have cataloged about half. But we know very few of the smaller
ones; there are probably considerably more than a million asteroids
in the 1 km range.

11 comets and asteroids have been explored by spacecraft so far, as follows: ICE flyby of Comet Giacobini-Zinner. Multiple flyby missions to Comet Halley. Giotto (retarget) to Comet Grigg-Skellerup. Galileo flybys of asteroids Gaspra and Ida (and Ida satellite Dactyl). NEAR-Shoemaker flyby of asteroid Mathilde on the way to orbit and land on Eros. DS-1 flybys of asteroid Braille and Comet Borrelly. Stardust flyby of asteroid Annefrank and recent sample collection from Comet Wild 2. For future we can expect: Hayabusa (MUSES-C) to asteroid Itokawa, Rosetta to Comet Churyumov-Gerasmenko, Deep Impact to Comet Tempel 1, and Dawn to orbit asteroids Vesta and Ceres.

243 Ida and 951 Gaspra
were photographed by the Galileo
spacecraft on its way to Jupiter.
The NEAR
mission flew by 253 Mathilde (left) on 1997 June 27
returning many images.
NEAR (now renamed "NEAR-Shoemaker") entered orbit around 433 Eros (right)
in January 1999 and returned a wealth of images and data. At the end of its mission
it actually landed on Eros.

The largest asteroid by far is 1 Ceres.
It is 974 km in diameter
and contains
about 25% of the mass of all the asteroids combined. The next largest are
2 Pallas, 4 Vesta and
10 Hygiea which are between 400 and 525 km in diameter. All
other known asteroids are less than 340 km across.

There is some debate as to the
classification of asteroids, comets and moons.
There are many planetary satellites that are probably better thought of as
captured asteroids. Mars's tiny moons
Deimos and
Phobos,
Jupiter's
outer eight moons,
Saturn's outermost moon,
Phoebe,
and perhaps some of the newly discovered moons of Saturn,
Uranus and Neptune
are all more similar to asteroids than to the larger moons.
(The composite image at the top of this page shows Ida, Gaspra, Deimos and Phobos
approximately to scale.)

C-type, includes more than 75% of known asteroids:
extremely dark (albedo 0.03);
similar to carbonaceous chondrite meteorites;
approximately the same chemical composition as the
Sun minus hydrogen, helium and
other volatiles;

Because of biases involved in the observations (e.g. the dark C-types
are harder to see),
the percentages above may not be representative of the true distribution of
asteroids. (There are actually several classification schemes in use today.)

There is little data about the densities
of asteroids. But by sensing the Doppler effect on radio waves
returning to Earth from NEAR owing to the (very slight)
gravitational tug between asteroid and spacecraft, Mathilde's mass
could be estimated. Surprisingly, its density turns out to be not
much greater than that of water, suggesting that it is not a solid
object but rather a compacted pile of debris.

Main Belt: located between Mars and Jupiter roughly
2 - 4 AU from the Sun;
further divided into subgroups: Hungarias, Floras,
Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are
named after the main asteroid in the group).

Trojans:
located near Jupiter's
Lagrange points (60 degrees
ahead and behind Jupiter in its orbit). Several hundred such
asteroids are now known; it is estimated that there may be a
thousand or more altogether. Curiously, there are
many more in the leading Lagrange point (L4) than in the trailing one (L5).
(There may also be a few small asteroids
in the Lagrange points of Venus and Earth
(see Earth's Second Moon)
that are also sometimes known as
Trojans;
5261 Eureka is a "Mars Trojan".)

There also a few "asteroids" (designated as "Centaurs")
in the outer solar system: 2060 Chiron
(aka 95 P/Chiron) orbits
between Saturn and Uranus;
the orbit of 5335 Damocles ranges from near Mars
to beyond
Uranus; 5145 Pholus orbits from Saturn to past
Neptune.
There are probably many more, but such planet-crossing orbits are unstable and
they are likely to be perturbed
in the future. The composition of these objects
is probably more like that of comets or
the Kuiper Belt objects than that of ordinary asteroids.
In particular, Chiron is now classified as a comet.

4 Vesta
has been studied recently with HST
(left). It is a
particularly interesting asteroid in that it seems to have been differentiated
into layers like the terrestrial planets.
This implies some internal heat source
in addition to the heat released by long-lived radio-isotopes which alone would be
insufficient to melt such a small object. There is also a gigantic impact basin
so deep that it exposes the mantle beneath Vesta's outer crust.

Though they are never visible with the unaided eye, many asteroids are visible with
binoculars or a small telescope.